The present invention relates to a method of bonding metal plates and, more particularly, to a method of bonding hot rolled plates which makes it possible to perform continuous rolling by bonding the hot rolled plates in a short time when the hot rolled plates are rolled by a set of coarse rolling mills and a set of finish rolling mills.
There has been a strong need to improve the productivity and the quality in the manufactured metal plates and to realize automatic operation in a hot rolling facility, by continuously performing finish rolling. The key technology is in the bonding of hot rolled plates (hereinafter, referred to as "sheet bars"). In the case of cold rolling plate, it is possible to bond the plates and perform continuous rolling because the rolled plates are thin and the strength of the bonding is sufficient. However, in the case of sheet bars, it is difficult to bond the bars by welding because the sheet bars are thick and the rolling speed (transferring speed of the rolled plate) is faster than that in a cold rolling mill, since the rolling must be finished before the temperature of the bars decreases.
In the past there have been many proposed methods of bonding sheet bars, such as an electric heating method, a gas heating method, a melt-chopping method and a friction method. However, these methods still have not proven to be satisfactory. The most important reason why they are not satisfactory is that it takes a long time to bond the sheet bars. In a running type bonding apparatus, where the bonding apparatus moves with the bars, if the bonding of sheet bars can not be performed in a short time, the running length of the bonding apparatus becomes excessive, since the rolling speed of bar is fast, as described above. Therefore, it is difficult to realize such a machine. On the other hand, in a case where the bonding apparatus is of the fixed type, a huge looper for accumulating bars is required, since the thickness of the sheet bar is generally as much as 20 to 50 mm. In the past, it took 20 to 30 seconds at a minimum to bond sheet bars, including the preparation for the bonding, the removal of roll-over and burrs caused by pressing, etc. Technologies for shortening the bonding time of bars are disclosed in Japanese Patent Application Laid-Open No. 4-187386 and Japanese Patent Application Laid-Open No. 6-39405.
In these technologies, strong metallurgical bonding can be obtained in such a manner that the end portions of a preceding rolled plate and the following rolled plate are cut and rubbed at the same time, and are butted against each other if necessary, to bring clean surfaces of the rolled plates into direct contact with each other without the intervention of the oxide films on the surfaces of the hot rolled plates.
As a method of completing bonding in a short time, there is a push back blanking method, for example, which is disclosed in fiscal Heisei 6 (fiscal 1994) Plastic Working Spring Season Proceedings, page 455. This is a method wherein materials to be bonded are overlapped on each other, held by a die and stripper, and then pressed in by a punch, whereby the materials are bonded. According to this method, the sheared surfaces of the materials are bonded in an instant, so that a high bonding strength can be attained.
From a series of tests in which the above technologies, as disclosed in JP A 4-187386 and JP A 6-39405, were applied to various kinds of materials of hot rolled plates having various thicknesses, the inventors of the present invention found that when the rolled plates were made of carbon steel or stainless steel and the thickness was large, the bonding strength was insufficient and a break sometimes occurred in the bonded portion during rolling. A break in a rolled plate during rolling scratches the surface of the roll, causing the whole process line to be stopped, which substantially decreases production efficiency.
Further, the push back blanking method is an application of blanking, using a punch and a die, and can not be used for bonding of metal plates, which are the objects to be bonded by the present invention, in particular, for bars each having a thickness of several tens of mm.